Radiation thermometry is a technique which allows the measurement of the temperature of a remote object by analyzing the radiation emitted by the object. All objects at temperatures greater than 0 kelvin emit radiation which can be measured to determine the temperature of the object. The concept of radiation thermometry is based upon the underlying principles that as an object which has a certain emissivity heats up, it emits radiation that shifts in wavelength and increases in intensity such that an object which radiates with an orange glow is hotter than an otherwise identical object which radiates with a red glow. Details of radiation thermometry are discussed in the literature (See, for example, Tenney; Mechanical Engineering, Oct. '86; "Red Hot . . . AND HOTTER", pp. 36-41.)
In the case of an object inside a furnace or other environment of elevated ambient temperature, it is not easy to optically detect the radiation from the object alone. Radiation from hot elements present in the environment will be reflected to some degree by the object being detected. Thus, where the object is reflecting significant amounts of unabsorbed radiation, simple optical detection apparatus will indicate a temperature which is higher than the true temperature of the object alone.